'Overachiever' often has a positive tone, suggesting admiration for someone's exceptional effort. Her classmates called her an overachiever because she always exceeded the teachers' expectations.
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Possibly Negative
Sometimes 'overachiever' can imply that someone is working too hard to gain approval or success. He's labeled an overachiever because he constantly seeks the boss's praise by working late.
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Comparative Context
Use 'overachiever' to compare somebody's performance against a group or standard. As an academic overachiever, Maria often surpassed her peers in grades and research.